Global Positioning System (GPS) is a valuable tool for self locating as well as tracking members of a group. The benefits of GPS are recognized in numerous military as well as civilian applications. In many situations, however, GPS signals are simply not available or, at best, intermittently observable. For example, in military or emergency rescue team applications, it is desirable for a team member to track the locations of his/her peers and to have a centralized location tracking the location of the team. Examples include a Marine fire team in a combat operation, or a rescue team entering a building or an underground cave. In these cases, extended absence of UPS is expected, which makes GPS-only tracking systems almost unusable.
A traditional approach to this problem relies on Inertia Measurement Units (IMU) and applies an appropriate reference frame transformation to estimate position. However, for low-cost commercial grade IMU, the bias in the accelerometer and gyroscope of the IMU produces substantial errors and renders this approach ineffective. Adding ranging sensors among group members using RF or ultrasonic devices has also been studied; these techniques may provide sufficient range accuracy, but are not able to provide accurate group navigation due to lack of directional information.